Recently, along with the popularity of compact cameras, single-lens reflex cameras and lens-fitted cameras is desired development of a silver halide light sensitive photographic material (hereinafter, also referred to as a photographic material) having high sensitivity and superior image quality. Accordingly, demand for improved performance of silver halide photographic emulsions has become stronger, and a high level demand for photographic performance such as enhanced sensitivity, superior graininess and sharpness have been raised.
In response to the demands, U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306 and 4,459,353 disclose a technique of using tabular silver halide grains (hereinafter, also simply denoted as tabular grains), thereby leading to advantages, such as enhancement of sensitivity, including enhancement of spectral sensitization efficiency with a sensitizing dye, an improvement of sensitivity/graininess, enhanced sharpness due to the specific optical property of tabular grains and enhanced covering power. However, these are still insufficient response to recent high level demands and still further enhanced performance is desired.
In connection with the trend in enhancement of sensitivity and image quality, the desire for enhanced pressure characteristics of a silver halide photographic material has also increased. Attempts to improve pressure characteristics by various means have been made, and the view that techniques of enhancing stress resistance of silver halide grains is more effective and preferable in practical use rather than technique of using additives such as a plasticizer, is now accepted. In response to such desire, emulsions comprised of core/shell type silver halide grains containing a high iodide silver iodobromide layer have been widely studied. Specifically, a silver iodobromide emulsion comprised of core/shell type grains having an internal phase containing 10 mol % or more iodide has been noted as an emulsion for use in color negative films.
U.S. Pat. No. 4,956,269 discloses a technique of introducing dislocation lines into tabular silver halide grains to enhance the sensitivity of a silver halide emulsion. It is generally known that application of pressure to silver halide grains results in fog formation or desensitization, and dislocation lines-introduced grains exhibit the problem that when subjected to pressure, marked desensitization occurs. JP-A 3-189642 (herein, the term, JP-A means an unexamined published Japanese Patent Application) discloses a monodispersed silver halide emulsion which is accounted for by tabular grains having an aspect ratio of 2 or more and containing 10 or more dislocation lines in fringe portions of the grain. However, such a technique did not improve marked pressure desensitization caused by introduction of dislocation lines.
JP-A 59-99433, 60-35726 and 60-147727, for example, disclose a technique of improving pressure characteristics with core/shell type grains. JP-A 63-220238 and 1-201649 disclose a technique of improving graininess, pressure characteristics and exposure intensity dependence as well as sensitivity. JP-A 6-235988 discloses a technique of enhancing pressure resistance by the use of multiple structure type monodispersed tabular grains having a high iodide intermediate shell.
There have been various factors of non-efficiency relating to the emulsion. As one of such factors is known recombination of a free electron with a positive hole. It has also been known that reduction sensitization is effective to prevent such a recombination described above. U.S. Pat. Nos. 2,487,850 and 2,512,925, and British Patent 789,823 disclose techniques for reduction sensitization. As was reported in Journal of Imaging Science Vol. 29, page 233 (1985), In light of the fact that sensitizing effects by reduction sensitization was rather lower than that of hydrogen sensitization in which a photographic material is treated under a hydrogen atmosphere, as was reported in Journal of Imaging Science Vol. 29, page 233 (1985), it is contemplated that further enhanced effects of reduction sensitization may be feasible.
There have been attempts of not only enhancing sensitivity but also improving other photographic characteristics such as fog, storage stability and latent image keeping. JP-A 1-196136 discloses the use of thiosulfonic acid compounds in combination with reduction sensitization, thereby leading to enhanced sensitivity/fog ratio. JP-A 8-15798 the combined use of a monodispersed silver halide emulsion and reduction sensitization, thereby leading to improvements in sensitivity, fog, graininess and latent image keeping. JP-A 1-127633 discloses a technique of occluding sulfur, selenium or tellurium ions within the grain through the design of halide composition of grains, whereby the sensitivity/fog ratio, pressure resistance and storage stability are improved. Thus, grain designing techniques of employing reduction sensitization in combination with other techniques enable to enhance effects of reduction sensitization and synergistically improve other characteristics.
The exact mechnism of reduction sensitization has not clearly been elucidated as yet. As is reported in Photographische Korrespondenz 1, 20 (1957) and Photographic Science and Engineering 19, 49 (1975), fine silver nuclei formed by reduction sensitization, that is, reduction sensitization nuclei contribute to sensitization through traping positive holes formed upon light absorption of silver halide and releasing electrons. According to Photographic Science and Engineering 16, 35 (1971) and ibid 23, 113 (1979), positive holes have property of trapping not only positive holes but also electrons and therefore the behavior of reduction sensitization nuclei cannot be accounted for only in terms of the positive hole trapping mechanism. Furthermore, in cases when aged under conditions of high temprature and high humidity, the behavior of reduction sensitization nuclei, for example, whether a reaction such as degradation or coagulation occurs or not, has not yet been proved.
Thus, techniques of designing constitution of silver halide grains, including conventional reduction sensitization are not obtained by completely understanding the behavior of the reduction sensitization nuclei and expecting synthetic characteristics of the emulsion, wherein there is clearly room for further improvement.